Session Transfer From A Circuit Switched Network To A Packet Switched Network

ABSTRACT

The invention relates to methods and nodes for supporting a session transfer, for a user equipment, from a circuit switched network to a packets witched network. According to one aspect a method for operating a control node of a core network is provided. The method comprises the steps: requesting from a radio access network of the circuit switched network information on whether a capability of the user equipment related to the session transfer are compatible with a network configuration related to the session transfer; receiving from the radio access network an indication whether the capability of the user equipment and the network configuration are compatible; determining whether to indicate that the session transfer is supported by using the received compatibility indication; and setting an indication whether the session transfer is supported according to the determination.

TECHNICAL FIELD

The invention relates to methods and nodes for supporting a sessiontransfer, such as a voice session, from a circuit switched network to apacket switched network. In particular it is focused on the support oftransfer of a session from UTRAN/GERAN to E-UTRAN/HSPA.

BACKGROUND

Single Radio Voice Call Continuity (SRVCC) is a functionality thatallows a VoIP (Voice over IP) or an IMS (IP Multimedia Subsystem) callin the packet domain, such as an LTE domain to be moved to a legacycircuit switched voice domain, such as GSM/UMTS or CDMA 1x.

As an example, SRVCC may function as follows: A user equipment (UE) thatis SRVCC capable and that accesses an LTE network is engaged in a voicecall. When the UE leaves the LTE coverage area it may be determined thatthe voice call is to be handed over to a legacy circuit switched domainand the voice call is then transferred from LTE to the circuit switchednetwork.

According to 3GPP TS 23.216 V11.4.0 Single Radio Voice Call Continuity(SRVCC) may relate to voice call continuity between IMS (IP MultimediaSubsystem) over PS (Packet Switched) access and CS (Circuit Switched)access for calls that are anchored in IMS when the UE (User Equipment)is capable of transmitting/receiving on only one of those accessnetworks at a given time.

Single Radio Video Call Continuity (vSRVCC) relates to video callcontinuity from E-UTRAN (evolved UMTS Terrestrial Radio Access Network)to UTRAN-CS for calls that are anchored in the IMS when the UE iscapable of transmitting/receiving on only one of those access networksat a given time. The term vSRVCC refers to Single Radio Video CallContinuity.

Single Radio Voice Call Continuity (SRVCC) is discussed and specified in3GPP TS 23.216 V11.4.0, entitled Single Radio Voice Call Continuity(SRVCC) and in 3GPP TS 23.237 V11.4.0, entitled IP Multimedia Subsystem(IMS) Service Continuity. IMS SIP (Session Initiation Protocol)signaling is discussed in 3GPP TS 23.228 V11.4.0, entitled IP MultimediaSubsystem. User Equipment Radio Capability Match procedures arediscussed in 3GPP TS 23.401 V11.1.0 entitled General Packet RadioService (GPRS) enhancements for Evolved Universal Terrestrial RadioAccess Network (E-UTRAN) access and in 3GPP TS 23.060 V11.1.0 entitledGeneral Packet Radio Service (GPRS), Service description.

While SRVCC often refers to the handover of a session from the packetswitched domain to the circuit switched domain, in this application theopposite direction of a session transfer is considered, i.e. sessiontransfer from the circuit switched domain to the packet switched domainwill be the main focus. The concept of SRVCC from circuit switched topacket switched will thus also be referred to as rSRVCC, meaning returnSRVCC or reverse SRVCC.

The current 3GPP specification TS23.216 V11.4.0 discusses in Section5.3.2b an MSC (Mobile Switching Centre) enhanced for UTRAN/GERAN toE-UTRAN/UTRAN (HSPA) SRVCC; (UTRAN: UMTS Terrestrial Radio AccessNetwork, GERAN: GSM EDGE Radio Access Network, E-UTRAN: evolved UTRAN,HSPA: High Speed Packed Access). According to this section, the MSC mayinform the RAN (Radio Access Network) about the possibility to performCS (Circuit Switched) to PS (Packet Switched) SRVCC by sending a “CS toPS SRVCC operation possible” to the RNC (Radio Network Controller) oralternatively to the BSC (Base Station Controller) only for normal TS 11call (i.e. not an emergency call—TS 12, 3GPP TS 22.003). The MSC Serverdetermines the possibility of performing CS to PS SRVCC based on: the CSto PS SRVCC capability of the UE, the presence of the CS to PS SRVCCallowed indication in the subscription data of the user, and the IMSregistration status of the UE.

According to existing specification, the MSC Server indicates to the RANthat rSRVCC (SRVCC from CS to PS) is supported even if the UE does notsupport the actually required capabilities. For example, the networksupports rSRVCC from UTRAN to LTE but the UE may only supports rSRVCCfrom GERAN to LTE. In such cases a handover or session transfer from theCS domain to the PS may fail or the call may even be lost or need to berecovered.

SUMMARY

Accordingly it is an object of the invention to obviate at least some ofthe above disadvantages and to provide improved methods and nodes for asession transfer from the circuit switched to the packet switcheddomain. The session transfer shall be reliable. Signaling overhead shallbe minimized.

According to an embodiment of the invention, a method for operating acontrol node of a core network and for supporting, for a user equipment,a session transfer from a circuit switched network to a packet switchednetwork is provided. The control node requests from a radio accessnetwork of the circuit switched network information on whether acapability of the user equipment related to the session transfer arecompatible with a network configuration related to the session transfer.The control node receives from the radio access network an indicationwhether the capability of the user equipment and the networkconfiguration are compatible. The control node determines whether toindicate that the session transfer is supported by using the receivedcompatibility indication. The control node sets an indication whetherthe session transfer is supported according to the determination.

According to an embodiment of the invention, a method for operating anode of a radio access network for supporting, for a user equipment, asession transfer from a circuit switched network to a packet switchednetwork is provided. The node receives from a control node of a corenetwork a request of information on whether a capability of the userequipment related to the session transfer is compatible with a networkconfiguration. The node checks whether the user equipment capability andthe network configuration are compatible. The node indicates to thecontrol node whether the user capability and the network configurationare compatible.

According to an embodiment of the invention, a control node for a corenetwork and for supporting, for a user equipment, a session transferfrom a circuit switched network to a packet switched network isprovided. The control node comprises a network interface to request froma radio access network of the circuit switched network information onwhether a capability of the user equipment related to the sessiontransfer is compatible with a network configuration related to thesession transfer and to receive from the radio access network anindication whether the capability of the user equipment and the networkconfiguration are compatible. The control node further comprises aprocessor configured to determine whether to indicate that the sessiontransfer is supported by using the received compatibility indication,and configured to set an indication whether the session transfer issupported according to the determination.

According to an embodiment of the invention, a node of a radio accessnetwork for supporting, for a user equipment, a session transfer from acircuit switched network to a packet switched network is provided. Thenode comprises a network interface for receiving, from a control node ofa core network, a request of information on whether a capability of theuser equipment related to the session transfer is compatible with anetwork configuration, and for indicating to the control node whetherthe user equipment capability and the network configuration arecompatible. The node comprises further a processor configured to checkwhether the user equipment capability and the network configuration arecompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an exemplary network architectureaccording to the state of the art.

FIG. 2 shows a flowchart for illustrating a method in a control node ofa core network according to one embodiment of the invention.

FIG. 3 shows a flowchart for illustrating a method in a radio accessnetwork according to one embodiment of the invention.

FIG. 4 shows a signaling diagram for illustrating signaling in anembodiment of the invention.

FIG. 5 schematically illustrates a control node of a core networkaccording to one embodiment of the invention.

FIG. 6 schematically illustrates a node of a radio access networkaccording to one embodiment of the invention.

FIG. 7 shows a flowchart for illustrating a method in a control node ofa core network according to an embodiment of the invention.

FIG. 8 shows a flowchart for illustrating a method in a user equipmentaccording to one embodiment of the invention.

FIG. 9 schematically illustrates a control node of a core networkaccording to one embodiment of the invention.

FIG. 10 schematically illustrates a user equipment according to oneembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, the invention will be explained in more detail byreferring to exemplary embodiments and to the accompanying drawings. Theillustrated embodiments relate to concepts for a session transfer from acircuit switched network to a packet switched network, e.g. from UTRANor GERAN to E-UTRAN or HSPA. This direction of session transfer is alsoreferred to as return or reverse direction. As far as a voice session isconcerned the return or reverse direction is referred to as rSRVCC(return/reverse SRVCC), or simply a CS to PS SRVCC.

FIG. 1 schematically illustrates an exemplary network architecture inwhich a session transfer from a circuit switched network to a packetswitched network can be performed. A similar structure is also presentedin 3GPP TS 23.216 V11.4.0, Section 5.2.4.

In the example according to FIG. 1, a user equipment 1 is at firstserved by the circuit switched access network 2, e.g. UTRAN or GERAN.The access network 2 is connected to an MSC server 3 which is connectedto an IMS 4. A session of the UE 1 is to be transferred from the circuitswitched network 2 to a packet switched network 5, e.g. aTarget-E-UTRAN. This example shows a bearer path 6 before the sessiontransfer and a bearer path 7 and a signaling path 8 after the sessiontransfer. After the session transfer, the bearer path 7 runs from the UE1 via the packet switched network 5 (Target E-UTRAN) and a Serving/PDNGateway 9 to the IMS 4. A Serving GPRS Support Node (SGSN) 12, aMobility Management Entity (MME) 10 and a Home Subscriber Server (HSS)11 may also be involved in signaling relevant for the session transfer.A core network in this example can be seen as the part of thearchitecture that resides between the circuit switched access network 2,the packet switched access network 5 and the IMS. The core network maygenerally be or comprise at least one of a circuit switched corenetwork, a packet switched core network or an evolved packet corenetwork. A network, in general, may comprise, depending on the context,at least one of a circuit switched access network, a packet switchedaccess network, a circuit switched core network, and a packet switchedcore network.

FIG. 2 shows a flowchart for illustrating a method in a control node ofa core network according to one embodiment. The method supports asession transfer from a circuit switched to a packet switched network. Auser equipment which is served by a radio access network of the circuitswitched network uses the session.

Examples of circuit switched networks are GSM based networks and UMTSbased networks. Each of which comprises an access network, e.g. GERANtype or UTRAN type, and a core network comprising several nodes such asan MSC server. An MSC server is an example of a control node of a corenetwork.

Examples of packet switched networks are LTE based networks (E-UTRAN)and HSPA. The mentioned access networks are connected to a correspondingcore network.

Session transfer comprises that a session is transferred or handed overfrom one access network or technology to another access network ortechnology. A session to be transferred can in particularly be a voicesession or a video session. In this sense, the session transfer allowsmobility of the user equipment between different access networks ortechnologies, including circuit switched access networks and packetswitched access networks. A voice call is a session that can betransferred between the mentioned access networks or technologies.

In a step S1, the control node requests from a radio access network ofthe circuit switched network information whether a capability of theuser equipment related to the session transfer are compatible with anetwork configuration related to the session transfer. A networkconfiguration may be an operator configuration or a configuration set byan operator of the network. The network may comprise radio accessnetwork(s) and core network(s). The network configuration may concern anetwork capability of a network operator. In one example, the networkcomprises a circuit switched access network, a packet switched accessnetwork and a core network. For example, the control node may direct therequest to a radio network controller (RNC) of a UTRAN or to a basestation controller (BSC) of a GERAN.

The capability of the user equipment may be an rSRVCC capability of theUE, which indicates that a UE is capable to be transferred from acircuit switched to a packet switched network. Examples are thecapability to be transferred from UTRAN to E-UTRAN, from GERAN toE-UTRAN, or from GERAN to HSPA. As several types of such access networksexist, e.g. TDD LTE or FDD LTE, the capability may also define whetherthe UE is capable to be transferred from a particular type ofUTRAN/GERAN to a particular type of E-UTRAN/HSPA. The capability of theUE may also be that the UE is capable for Voice over a PS network suchas UTRAN/E-UTRAN.

The capability of the user equipment may also or alternatively comprisethe information whether the UE is capable for an FDD mode or a TDD modeor both of GERAN, UTRAN or E-UTRAN.

The capability of the user equipment may also or alternatively compriseat least one piece of information of the following group of pieces ofinformation: the UE supports or does not support a certain frequencyband of the GERAN/UTRAN and/or E-UTRAN, FDD and/or TDD for UTRAN, FDDand/or TDD for E-UTRAN, rSRVCC for GERAN to FDD and/or TDD HSPA, rSRVCCfrom FDD and/or TDD UTRAN to FDD/TDD HSPA, rSRVCC for GERAN to FDDand/or TDD E-UTRAN, and rSRVCC from FDD and/or TDD UTRAN to FDD/TDDE-UTRAN.

It is noted that not all possible information are mentioned that definethe UE capability, however the skilled person will understand from theexamples given above that the capability of the UE may comprise any kindof information that is related to a session transfer capability. Thecapability information may answers the question whether a certainmobility feature is supported, i.e. the transfer of a session from onenetwork to another.

If the UE shows a certain capability and the network is also configuredto support the capability or the network indicates support of thecertain capability or mobility feature, than the network configurationrelated to session transfer and the capability of UE related to thesession transfer match or are compatible.

For example, if the network is configured to support a particular rSRVCCfor the UE or indicates that the particular rSRVCC of the UE issupported and if also the capability of the UE indicates that theparticular rSRVCC is supported, than the UE capability and the networkconfiguration match or are compatible. If the UE is not capable tosupport the particular rSRVCC than the UE capability and the rSRVCC donot match.

The control node may select a certain capability and indicate it to theradio access network. For example, the control node may be configured tosupport several types of session transfer, such as from GERAN to HSPA,from UTRAN to TDD-LTE and from UTRAN to FDD-LTE. When due to certaincircumstances the control node is only interested whether a UE supportsUTRAN to TDD-LTE than the control node may indicate that selectedcapability to the RAN. In this case not the entire capabilityinformation needs to be checked and signaled but only the selectedcapability is checked and signaled.

In step S2, the control node receives from the radio access network anindication whether the capabilities of the user equipment and thenetwork configuration are compatible. This information can be a simple“yes” or “no.”

In step S3, the control node determines whether to indicate that thesession transfer is supported by using the received compatibilityindication. Besides the received compatibility information the controlnode may take further information into account, e.g. information onnetwork load, cell load, rSRVCC capability indicated by the HSS,priority or other information. The further information may be obtainedfrom the network. Typically, the control node determines that sessiontransfer is supported only if the compatibility indication is indicatingthat rSRVCC is supported and if none of the other information preventsthe use of rSRVCC.

The control node can thus e.g. determine that the session transfer isnot supported although the control node received the indication from theradio access network that the capabilities related to the sessiontransfer of the UE are compatible with the network configuration.

In step S4, the control node sets an indication whether the sessiontransfer is supported according to the determination. The control nodemay send the set indication to the radio access network.

The above described method can be seen as a match procedure between thecontrol node of the core network and the RAN (radio access network) tomatch the capability of a UE served by the RAN and the network in viewof a possible session transfer. Whether the control node finallyindicates that the session transfer is supported depends on the outcomeof the match procedure and may also depend on further informationavailable at the control node.

The actual session transfer will only be performed if the control nodehas set the indication that the session transfer is supported. In thisway it can be assured that the session transfer can only be performed ifthe user equipment and the network have matching capabilities related tothe session transfer.

FIG. 3 shows a flowchart for illustrating a method in a radio accessnetwork according to one embodiment. The method may run in a node of aradio access network, such as an RNC (Radio Network Controller) in UTRANor a BSC (Base Station Controller) in GERAN.

The method supports, for a user equipment, a session transfer from acircuit switched network to a packet switched network. In a step S11, anode in the radio access network receives from a control node of a corenetwork a request of information on whether capabilities of the userequipment related to the session transfer are compatible with a networkconfiguration.

Upon receiving the request, the node checks in a step S14 whether theuser equipment capability and the network configuration are compatible.Essentially, the RAN needs to check whether a mobility feature issupported by the user equipment. For example, the RAN checks whether theUE capability indicates that the UE supports rSRVCC or a particular typeof rSRVCC.

When the RAN receives an indication, e.g. from the control node of thecore network, that the check shall only be performed for a particularcapability such as a particular rSRVCC capability, then the RAN performsonly the check(s) necessary for this capability. By indicating a certaincapability, the number of checks to be preformed is reduced and only thechecks required by the network and requested by the control node areperformed.

The RAN may already know the UE capabilities and can thus perform therequested checks right away. In case the RAN has not already receivedthe capabilities of the UE, the RAN, in an optional step S12, requeststhe UE to upload or provide its capability information. With therequest, the RAN may indicate to the UE at least one particularcapability, the RAN is interested in. For example the RAN may indicatethat it is interested in the rSRVCC capability of the UE or a certaintype of SRVCC capability. The UE than provides the indicated capabilityinformation only. In this way the number of capability checks is reducedto the number which is of importance to the network and the amount ofdata to be uploaded from the UE is correspondingly reduced.

The capability information uploaded by the UE may comprise various typesof information relating to the UE capability in general. They maycomprise all capability information related to session transfer or to atleast one particular capability as explicitly requested by the RAN.

In a step S13, the radio access network receives the requestedcapability information of the UE. Steps S12 and S13 are optional as therequired capability information may already be available at the RAN. Thesteps may be performed if it is determined that the information requiresan update. Steps S12 and S12 are performed in case the capabilityinformation is not already available at the RAN or the capabilityinformation needs to be updated. Thus the RAN may check first whetherthe information is available or whether the information requires anupdate before performing steps S12 and S13.

In a step S15, the RAN indicates to the control node whether the UEcapability and the network configuration are compatible. For example,the RAN may indicate for rSRVCC to TDD-LTE “yes” whereas it indicatesfor rSRVCC to FDD-LTE “no”, meaning that the session transfer from thecurrent circuit switched network to TDD-LTE is supported and to FDD-LTEis not supported.

FIG. 4 shows a signaling diagram for illustrating signaling in oneembodiment. An MSC 31 of a core network is connected to a RAN 32.Messages between the MSC 31 and the RAN 32 may be exchanged via arespective interface such as the A-interface or the Iu interface. A UE33 is served by the RAN 32. It may be assumed that the UE is registeredin IMS before rSRVCC takes place. The UE may be registered for a voicesession. If the UE is registered for another service a re-registrationmay be necessary when the UE is handed over to the packet switchednetwork. A function T-ADS which is a Terminating Access Domain Selectionmay be used to ensure that terminated calls are routed correctly, evenif camping on a circuit switched network (e.g. a 2G or 3G network). Sucha function can ensure terminating on the packet switched networks onlyif the packet switched access supports IMS voice service.

If the MSC 31 requires more information on the UE capability to be ableto set the rSRVCC Indication(s), the MSC 31 may send a “UE RadioCapability Match Request” to the RAN 32.

This procedure is typically used during the Initial Attach procedure,during Location Area Update (LAU) procedure for the “first LAU followingGERAN Attach” or for “UE Capability update” (e.g. during an active CScall) or when MSC has not received the Voice Support Match Indicatorfrom the MME (e.g. during SRVCC), SGSN (e.g. during SRVCC) or other MSC(e.g. during MTRF procedure, Mobile Terminating Roaming Forwarding).

In step 35, the MSC 31 indicates whether the MSC 31 wants to receive a“Voice Support Match Indicator”. The MSC 31 may indicate whether the RAN32 should only check for rSRVCC.

In step 36, upon receiving a “UE Radio Capability Match Request” fromthe MSC 31, if the RAN 32 has not already received the UE capabilitiesfrom the UE 33 the RAN 32 requests the UE 33 to upload the UE Capabilityinformation by sending the “RRC UE Capability Enquiry”.

In step 37, the UE 33 provides the RAN 32 with its UE capabilitiessending the “RRC UE Capability Information”.

In step 38, upon receiving the “UE Radio Capability Match Request fromthe MSC” of step 35, the RAN 32 checks whether the UE capabilities arecompatible with the network configuration. If requested by the MSC 31,or if configured to do so, the RAN 32 only checks for the rSRVCCcapabilities.

In step 39, the RAN 32 provides a “Voice Support Match Indicator” to theMSC 31 to indicate whether the UE capabilities and networksconfiguration are compatible.

For determining the appropriate “UE Radio Capability Match Response”,the RAN 32 is configured, e.g., by an operator to check whether the UE33 supports certain capabilities required for IMS PS Voice continuity.In a shared network, the RAN 32 keeps a configuration separately perPLMN (Public Land Mobile Network).

The checks which are to be performed depend on network configuration. Inthe following some examples of UE capabilities are given: theSRVCC/rSRVCC, and UTRAN/E-UTRAN Voice over PS capabilities; the Radiocapabilities for GERAN/UTRAN/E-UTRAN FDD and/or TDD; and/or the supportof GERAN/UTRAN/E-UTRAN frequency bands.

The MSC 31 stores the received Voice support match indicator in the MMContext (MM: Mobility Management) and uses it as an input for settingthe rSRVCC Indication.

The above example can be analogous done for the case of LTE, where theMME makes a query on the capability match of the UE. The rSRVCCcapability indication would be provided by the UE to the MME as responseto the query of the capability match and then it would be provided bythe MME to the MSC Server during SRVCC from LTE to 2G/3G.

FIG. 5 schematically illustrates exemplary structures for implementingthe above-described concepts in a control node 41 for a core network.The control node 41 for a core network supports, for a user equipment, asession transfer from a circuit switched network to a packet switchednetwork. The control node 41 comprises a network interface 42 to requestfrom a radio access network of the circuit switched network informationwhether a capability of the user equipment related to the sessiontransfer are compatible with a network configuration related to thesession transfer and to receive from the radio access network anindication whether the capability of the user equipment and the networkconfiguration are compatible. Examples of the interface 42 are the Ainterface or the Iu-CS interface. Examples of the control node 41 are anMSC or an MME.

Further, the control node 41 includes a processor 44 coupled to thenetwork interface 42 and a memory 45 coupled to the processor 44.

The memory 45 may include a read-only memory (ROM), e.g., a flash ROM, arandom-access memory (RAM), e.g., a Dynamic RAM (DRAM) or a static RAM(SRAM), a mass storage, e.g., a hard disc drive or a solid state disk,or the like. The memory 45 includes suitably configured program code tobe executed by the processor 44 so as to implement the above-describedfunctionalities of the control node 41.

It is to be understood that the structure as illustrated in FIG. 5 ismerely schematic and that the control node 41 may actually includefurther components which, for the sake of clarity, have not beenillustrated, e.g., further interfaces. Also, it is to be understood thatthe memory 45 may include further types of program code modules, whichhave not been illustrated. For example, the memory 45 may includeprogram code modules for implementing typical functionalities of arespective control node.

FIG. 6 schematically illustrates exemplary structures for implementingthe above-described concepts in a node of a radio 51 access network,e.g. a BSC or an RNC.

The node 51 of a radio access network is adapted to support, for a userequipment, a session transfer from a circuit switched network to apacket switched network. In the illustrated structure, the node 51comprise a network interface 52 for receiving, from a control node of acore network, a request of information on whether a UE capabilityrelated to the session transfer are compatible with a networkconfiguration, and for indicating to the control node whether the usercapability and the network configuration are compatible.

Further, the node 51 comprises a processor 54 configured to checkwhether the user equipment capability and the network configuration arecompatible; The processor 54 is coupled to the network interface 52, toa radio interface 53 for requesting the user equipment to provide itscapability information and for receiving the capability information ofthe UE and to a memory 55. The memory 55 may include a read-only memory(ROM), e.g., a flash ROM, a random-access memory (RAM), e.g., a DynamicRAM (DRAM) or static RAM (SRAM), a mass storage, e.g., a hard disk orsolid state disk, or the like. The memory 55 includes suitablyconfigured program code to be executed by the processor 54 so as toimplement the above-described functionalities of the node 51.

It is to be understood that the structure as illustrated in FIG. 5 ismerely schematic and that the node 51 of a radio access network mayactually include further components which, for the sake of clarity, havenot been illustrated, e.g., further interfaces. Also, it is to beunderstood that the memory 55 may include further types of program codemodules, which have not been illustrated. For example, the memory 55 mayinclude program code modules for implementing typical functionalities ofa respective control node.

According to some embodiments, also a computer program product may beprovided for implementing concepts according to embodiments of theinvention, e.g., a computer-readable medium storing the program codeand/or other data to be stored in the memory 45 or the memory 55.

With the above described concepts it can be assured that a sessiontransfer from a circuit switched to a packet switched network is onlyperformed when the UE and the network have matching capabilities. Theconcepts relate, e.g., to a procedure between a control node of a corenetwork and a RAN to match capabilities regarding session transfer, to aRAN that determines session transfer capability from a circuit switchedto a packet switched network upon request of the control node, and to acontrol node determining whether to indicate that session transfer issupported.

It is to be understood that the examples and embodiments as explainedabove are merely illustrative and susceptible to various modifications.For example, the concepts could be used in other types of mobilecommunication networks, not explicitly mentioned so far. Further, it isto be understood that the above concepts may be implemented by usingcorrespondingly designed software in existing nodes, or by usingdedicated hardware in the respective nodes.

FIGS. 7 to 10 concern a registration of a user equipment in an IMS inthe context of rSRVCC as described so far. Embodiments related to userequipment registration described in the following sections can beconsidered independently of the already discussed embodiments and alsoin combination with previously described embodiments.

The technical standard specification 3GPP TS 23.237 V11.4.0, entitled IPMultimedia Subsystem (IMS) Service Continuity, discusses IMSregistration and a session transfer from circuit switched to packetswitched networks from various perspectives. In Section 6.1.3 of 3GPP itis stated that as a prerequisite for CS to PS SRVCC to be applied, theUE shall be IMS registered over PS. Consequently, the UE needs to be IMSregistered before rSRVCC can take place.

The user equipment should be registered for voice, otherwise it wouldhave to re-register when being handed over to e.g. LTE. A TerminatingAccess Domain Selection (T-ADS) ensures to terminate the IMS voicesession on a PS access if the PS access supports IMS voice.

Currently, the user equipment has to register into IMS on e.g. via 2G/3Gnetworks, e.g. GERAN or UTRAN, even if that access does not support IMSVoice over the PS domain. It is an object to improve existing IMSregistration concepts and nodes in the context of rSRVCC.

According to one embodiment a method in a control node of a core networkfor supporting registration of a user equipment in an Internet ProtocolMultimedia Subsystem, IMS, is provided. The method comprises the steps:checking whether a session transfer from a circuit switched network to apacket switched network is supported; and if the session transfer issupported, indicating to the user equipment that the session transfer issupported.

According to one embodiment a method in a user equipment forregistration of the user equipment in an Internet Protocol MultimediaSubsystem, IMS, is provided. The method comprises the steps: receiving,from a control node of a core network, an indication that a sessiontransfer from a circuit switched network to a packet switched network issupported; checking the received indication; and, if the sessiontransfer is supported, the user equipment registering in the IMS ormaintaining its IMS registration.

According to one embodiment a control node of a core network forsupporting registration of a user equipment in an Internet ProtocolMultimedia Subsystem, IMS, is provided. The control node comprises aprocessor for checking whether a session transfer from a circuitswitched network to a packet switched network is supported and a networkinterface for indicating to the user equipment that the session transferis supported, if the session transfer is supported.

According to one embodiment a user equipment for registration of theuser equipment in an Internet Protocol Multimedia Subsystem, IMS, isprovided. The user equipment comprises a radio interface for receivingan indication that a session transfer from a circuit switched network toa packet switched network is supported and a processor configured tocheck the received indication and to initiating registration of the userequipment in the IMS or to maintain the IMS registration of the userequipment.

FIG. 7 shows a flowchart of a method in a control node of a core networkaccording to one embodiment. The node may be a Mobile Switching Center(MSC) or a Mobility Management Entity (MME), or in general a function inthe core network. The core network is connected with a radio accessnetwork that serves a user equipment. The method supports theregistration of user equipment in an Internet Protocol MultimediaSubsystem, IMS, by providing relevant information to the user equipment.

In a step S71, the control node checks whether a session transfer from acircuit switched network to a packet switched network is supported. Anexample network is composed of a circuit switched access network, acircuit switched core network, a packet switched access network, and apacket switched core network. The core network(s) connects the accessnetwork(s) with an IMS. An example of a circuit switched networks areGERAN or UTRAN, which are circuit switched access networks. An exampleof a packet switched network is an E-UTRAN (evolved UTRAN). It ischecked whether the support of a session transfer is supported within anetwork, at least comprising the circuit switched and the packetswitched network and a core network. A session supported to betransferred can, for example, be a voice call or video call.

Examples of a session transfer from a circuit switched to packetswitched network are: rSRVCC from GERAN to FDD and/or TDD HSPA, rSRVCCfrom FDD and/or TDD UTRAN to FDD/TDD HSPA, rSRVCC for GERAN to FDDand/or TDD E-UTRAN, and rSRVCC from FDD and/or TDD UTRAN to FDD/TDDE-UTRAN.

In a step S72, the control node indicating to the user equipment thatthe session transfer is supported, if the check revealed that thesession transfer is supported by the network. The control node may sendthis information to a radio access network which forwards theinformation to the user equipment. The indication may be sent in aresponse to an Attach Request Message sent by the user equipment.

In this way the information whether the network supports sessiontransfer is made available to the user equipment.

FIG. 8 shows a flowchart of a method in a user equipment according toone embodiment. The method registers the user equipment in an InternetProtocol Multimedia Subsystem.

In a step S81, the user equipment receives from a control node of a corenetwork, an indication that a session transfer from a circuit switchednetwork to a packet switched network is supported. The receivedindication in step S81 corresponds to the indication of step S72 in FIG.7.

In step S82, the received indication is checked by the user equipmentand when the check reveals that the session transfer is supported, theuser equipment will in a step S83 register in the IMS or maintain itsIMS registration in case the registration is already in existence.

Typically the IMS registration is done via the packet switched network.The registration can in particularly be made for a voice session. Theregistration is also made if there is no indication that the session,such as a voice session, is supported in the packet switched network orif an indication explicitly states that the session is not supported bythe packet switched access network.

With respect to FIG. 7 embodiments from a control node perspective aredescribed, whereas with respect to FIG. 8 embodiments from a userequipment perspective are described.

The following example further illustrates various embodiments. A userequipment is currently camping on a 2G or 3G network, e.g. GERAN orUTRAN. The user equipment receives the indication that session transferfrom circuit switched network to a packet switched network (rSRVCC) issupported from a MSC Server. The indication is received from the MSC inresponse to an Attach Request message previously send by the userequipment. The user equipment registers in the IMS. The registration isdone even if there is no indication saying that IMS Voice is supportedfrom the packet switched network or if there is an indication statingthat IMS voice is not supported. Alternatively, the user equipmentmaintains its IMS registration that was initiated on an IMS Voicecapable access or on 2G/3G. For this example it is important to note,that T-ADS (Terminating Access Domain Selection) in the IMS ensures thatterminated calls are not routed to PS access if the PS access does notsupport IMS voice over PS. Embodiments suggest that the MSC Serverinforms the UE that rSRVCC is possible and that a user equipmentperforms IMS registration for voice also when currently used PS accessdoes not support IMS Voice over PS. With it the UE is registered in theIMS on 2G/3G for voice also when the currently used access does notsupport IMS voice.

FIG. 9 schematically shows exemplary structures for implementing theabove-described concepts in a control node 91 for a core network. Thecontrol 91 is configured to support a registration of a user equipmentin an Internet Protocol Multimedia Subsystem, IMS. The control node 91comprises a processor 94 for checking whether a session transfer from acircuit switched network to a packet switched network is supported, anda network interface 92 for indicating to the user equipment that thesession transfer is supported, if the session transfer is supported.Further the control node includes a memory 45 coupled to the processor.

The memory 95 may include a read-only memory (ROM), e.g., a flash ROM, arandom-access memory (RAM), e.g., a Dynamic RAM (DRAM) or a static RAM(SRAM), a mass storage, e.g., a hard disc drive or a solid state disk,or the like. The memory 95 includes suitably configured program code tobe executed by the processor 94 so as to implement the above-describedfunctionalities of the control node 91.

It is to be understood that the structure as illustrated in FIG. 9 ismerely schematic and that the control node 91 may actually includefurther components which, for the sake of clarity, have not beenillustrated, e.g., further interfaces. Also, it is to be understood thatthe memory 95 may include further types of program code modules, whichhave not been illustrated. For example, the memory 95 may includeprogram code modules for implementing typical functionalities of arespective control node.

FIG. 10 schematically shows exemplary structures for implementing theabove-described concepts in a user equipment 101. The user equipment isconfigured for registration in an Internet Protocol MultimediaSubsystem, IMS. The user equipment 101 comprises a radio interface 103for receiving an indication that a session transfer from a circuitswitched network to a packet switched network is supported; and aprocessor 104 configured to check the received indication and toinitiating registration of the user equipment in the IMS or to maintainthe IMS registration of the user equipment. The processor 104 is coupledto a memory 105.

The memory 105 may include a read-only memory (ROM), e.g., a flash ROM,a random-access memory (RAM), e.g., a Dynamic RAM (DRAM) or static RAM(SRAM), a mass storage, e.g., a hard disk or solid state disk, or thelike. The memory 105 includes suitably configured program code to beexecuted by the processor 104 so as to implement the above-describedfunctionalities of the node user equipment 101

It is to be understood that the structure as illustrated in FIG. 10 ismerely schematic and that the user equipment 101 may actually includefurther components which, for the sake of clarity, have not beenillustrated, e.g., further interfaces. Also, it is to be understood thatthe memory 105 may include further types of program code modules, whichhave not been illustrated. For example, the memory 105 may includeprogram code modules for implementing typical functionalities of theuser equipment 101.

According to some embodiments, also a computer program product may beprovided for implementing concepts according to embodiments of theinvention, e.g., a computer-readable medium storing the program codeand/or other data to be stored in the memory 95 or the memory 105.

It is to be understood that the examples and embodiments as explainedabove are merely illustrative and susceptible to various modifications.For example, the concepts could be used in other types of mobilecommunication networks, not explicitly mentioned so far. Further, it isto be understood that the above concepts may be implemented by usingcorrespondingly designed software in existing nodes, or by usingdedicated hardware in the respective nodes.

1-35. (canceled)
 36. A method for operating a control node of a corenetwork for supporting, for a user equipment, a session transfer from acircuit switched network to a packet switched network, the methodcomprising: the control node requesting, from a radio access network ofthe circuit switched network, information whether a capability of theuser equipment related to the session transfer is compatible with anetwork configuration related to the session transfer; the control nodereceiving, from the radio access network, an indication whether thecapability of the user equipment and the network configuration arecompatible; the control node determining whether to indicate that thesession transfer is supported based on the received compatibilityindication; and the control node setting an indication whether thesession transfer is supported according to the determination.
 37. Themethod of claim 36, wherein the determining whether to indicate that thesession transfer is supported is further based on further informationavailable at the control node.
 38. The method of claim 36, wherein thecircuit switched network comprises a UTRAN or a GERAN.
 39. The method ofclaim 36, wherein the packet switched network comprises an E-UTRAN or aHSPA network.
 40. The method of claim 36, wherein the capability of theuser equipment comprises a return Single Radio Voice Call Continuity(rSRVCC) capability.
 41. The method of claim 36, wherein the capabilityof the user equipment comprises a capability of the user equipment tosupport least one of: a frequency division duplex (FDD) mode of thecircuit switched network and/or the packet switched network; a timedivision duplex (TDD) mode of the circuit switched network and/or thepacket switched network; both a FDD mode and a TDD mode of the circuitswitched network and/or the packet switched network.
 42. The method ofclaim 36, wherein the capability of the user equipment comprises acapability of the user equipment to support a certain frequency band ofthe circuit switched network and/or the packet switched network.
 43. Themethod of claim 36, wherein the control node requesting the informationcomprises the control node indicating a selected capability of the userequipment.
 44. A method for operating a first node of a radio accessnetwork for supporting, for a user equipment, a session transfer from acircuit switched network to a packet switched network, the methodcomprising: the first node receiving, from a control node of a corenetwork, a request of information on whether a capability of the userequipment related to the session transfer is compatible with a networkconfiguration; the first node checking whether the user equipmentcapability and the network configuration are compatible; the first nodeindicating, to the control node, whether the user equipment capabilityand the network configuration are compatible.
 45. The method of claim44, further comprising: the first node requesting the user equipment toprovide its capability information; the first node receiving thecapability information of the user equipment.
 46. The method of claim45, further comprising the first node indicating, to the user equipment,a selected capability of the user equipment related to the sessiontransfer.
 47. The method of claim 44, wherein the capability of the userequipment comprises a capability of the user equipment to support leastone of: a frequency division duplex (FDD) mode of the circuit switchednetwork and/or the packet switched network; a time division duplex (TDD)mode of the circuit switched network and/or the packet switched network;both a FDD mode and a TDD mode of the circuit switched network and/orthe packet switched network.
 48. The method of claim 44, wherein thecapability of the user equipment comprises a capability of the userequipment to support a certain frequency band of the circuit switchednetwork and/or the packet switched network.
 49. A control node for acore network and for supporting, for a user equipment, a sessiontransfer from a circuit switched network to a packet switched network,the control node comprising: a network interface configured to: request,from a radio access network of the circuit switched network, informationon whether a capability of the user equipment related to the sessiontransfer are compatible with a network configuration related to thesession transfer; and receive from the radio access network anindication whether the capability of the user equipment and the networkconfiguration are compatible; a processor configured to: determinewhether to indicate that the session transfer is supported based on thereceived compatibility indication; and set an indication whether thesession transfer is supported according to the determination.
 50. Thecontrol node of claim 49, wherein the processor is configured todetermine whether to indicate that the session transfer is supportedfurther based on further information available at the control node. 51.The control node of claim 49, wherein the circuit switched networkcomprises an UTRAN or a GERAN.
 52. The control node of claim 49, whereinthe packet switched network comprises an E-UTRAN or a HSPA network. 53.A node of a radio access network for supporting, for a user equipment, asession transfer from a circuit switched network to a packet switchednetwork, the node comprising: a network interface configured to:receive, from a control node of a core network, a request of informationon whether a capability of the user equipment related to the sessiontransfer is compatible with a network configuration; and indicate, tothe control node, whether the user capability and the networkconfiguration are compatible; and a processor configured to checkwhether the user equipment capability and the network configuration arecompatible.
 54. A computer program product stored in a non-transitorycomputer readable medium for operating a control node of a core networkfor supporting, for a user equipment, a session transfer from a circuitswitched network to a packet switched network, the computer programproduct comprising software instructions which, when run on a processorof the control node, causes the control node to: request, from a radioaccess network of the circuit switched network, information whether acapability of the user equipment related to the session transfer arecompatible with a network configuration related to the session transfer;receive, from the radio access network, an indication whether thecapability of the user equipment and the network configuration arecompatible; determining whether to indicate that the session transfer issupported based on the received compatibility indication; and setting anindication whether the session transfer is supported according to thedetermination.
 55. A computer program product stored in a non-transitorycomputer readable medium for operating a first node of a radio accessnetwork for supporting, for a user equipment, a session transfer from acircuit switched network to a packet switched network, the computerprogram product comprising software instructions which, when run on aprocessor of the first node, causes the first node to: receive, from acontrol node of a core network, a request of information on whether acapability of the user equipment related to the session transfer iscompatible with a network configuration; check whether the userequipment capability and the network configuration are compatible;indicate, to the control node, whether the user equipment capability andthe network configuration are compatible.